Are you looking for Contacts in the Mail app for Windows 10?
The Mail app uses the People app for Windows 10 to store contact information. When you open a new message in Mail for Windows 10, start typing a contact's name or email address and Mail for Windows 10 will search the People app and display a list of suggestions for you to choose from. If the program can't find the right person, you can choose Search Directory.
If you add an Outlook.com account to Mail for Windows 10, your Outlook.com contacts are automatically stored in the People app.
Send an email to a contact stored in the People app
If you know the contact's name or email address, you can start typing the first few letters of their name or email address and Mail for Windows 10 will suggest matching contacts.
Choose a contact to add their email address to the To line of your email. If no match is found or the suggestions don't include the address you need, choose Search Directory.
Send an email to a contact from the People app
You can also send an email directly from the People app.
In the lower left corner of Windows 10, choose the Start button .
Begin typing People, and in the left pane, when Windows suggests the People app, choose the app to open it.
In the top left Search box, type the name of the person you want to send an email message to.
Windows will suggest one or more matches. Choose the one you want to send the email message to.
In the person's contact profile, click their email address.
Note: The contact name you choose must have a valid email address.
When you are prompted to choose an email app, click Mail. If it applies, check the Always use this app box.
If you have several accounts associated with your Mail app, select the account you want to send the email message from.
In Mail, fill in the Subject line and the body of the message, and click Send.